Radio plasma imager simulations and measurements

The Radio Plasma Imager (RPI) will be the first-of-its kind instrument designed to use radio wave sounding techniques to perform repetitive remote sensing measurements of electron number density (N _e) structures and the dynamics of the magnetosphere and plasmasphere. RPI will fly on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission to be launched early in the year 2000. The design of the RPI is based on recent advances in radio transmitter and receiver design and modern digital processing techniques perfected for ground-based ionospheric sounding over the last two decades. Free-space electromagnetic waves transmitted by the RPI located in the low-density magnetospheric cavity will be reflected at distant plasma cutoffs. The location and characteristics of the plasma at those remote reflection points can then be derived from measurements of the echo amplitude, phase, delay time, frequency, polarization, Doppler shift, and echo direction. The 500 m tip-to-tip X and Y (spin plane) antennas and 20 m Z axis antenna on RPI will be used to measures echoes coming from distances of several R _E. RPI will operate at frequencies between 3 kHz to 3 MHz and will provide quantitative N _e values from 10^–1 to 10⁵ cm^–3. Ray tracing calculations, combined with specific radio imager instrument characteristics, enables simulations of RPI measurements. These simulations have been performed throughout an IMAGE orbit and under different model magnetospheric conditions. They dramatically show that radio sounding can be used quite successfully to measure a wealth of magnetospheric phenomena such as magnetopause boundary motions and plasmapause dynamics. The radio imaging technique will provide a truly exciting opportunity to study global magnetospheric dynamics in a way that was never before possible.     

Investigations of irregularities in remote plasma regions by radio sounding: applications of the radio plasma imager on image

The Radio Plasma Imager (RPI) on the IMAGE mission operates like a radar by transmitting and receiving coherent electromagnetic pulses. The RPI is designed to receive mirror-like (specular) reflections and coherent scatter returns. Long-range echoes of electromagnetic sounder waves are reflected at remote plasma cutoffs. Thus, analyses of RPI observations will yield the plasma parameters and distances to the remote reflection points. The RPI will employ pulse compression and spectral integration techniques, perfected in ground-based ionospheric digital sounders, in order to enhance the signal-to-noise ratio in long-range magnetospheric sounding. When plasma irregularities exist in the remote magnetospheric plasmas being probed by the sounder waves, echo signatures may become complicated. Experience in ionospheric sounding under such conditions indicates that sounding echo strengths can actually be enhanced by the presence of irregularities, and ground-based sounding indicates that coherent detection techniques can still be employed. In this paper we investigate the conditions that will allow coherent signals to be detected by the RPI and the signatures of scattering to be expected in the presence of multi-scale irregularities. Sounding of irregular plasma structures in the plasmasphere, plasmapause and magnetopause are also discussed.     

The First two Years of Image

The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) is the first satellite mission that is dedicated to imaging the Earth's magnetosphere. Using advanced multispectral imaging techniques along with omnidirectional radio sounding, IMAGE has provided the first glimpses into the global structure and behavior of plasmas in the inner magnetosphere. Scientific results from the two-year prime mission include the confirmation of the theory of plasmaspheric tails and the discovery of several new and unpredicted features of the plasmasphere. Neutral-atom imaging has shown how the ring current develops during magnetic storms and how ionospheric ions are injected into the ring current during substorms. The first global imaging of proton auroras has allowed the identification of the ionospheric footprint of the polar cusp and its response to changes in the interplanetary magnetic field. Detached subauroral proton arcs have been found to appear in the afternoon sector following south-north and east-west rotations of the IMF. Low-energy neutral atom imaging has shown global-scale ionospheric outflow to be an immediate response to solar-wind pressure pulses. Such imaging has also provided the first measurements of solar wind and interstellar neutral atoms from inside the magnetosphere. Radio sounding has revealed the internal structure of the plasmasphere and identified plasma cavities as the source of kilometric continuum radiation. These and numerous other scientific results now set the stage for the extended mission of IMAGE in which the imaging perspective will change markedly owing to orbital evolution while the magnetospheric environment undergoes a transition from solar maximum toward solar minimum.     

IMAGE mission overview

The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission is the first mission in NASA's MIDEX (Mid-size Explorer) program. It is the first satellite mission that is dedicated to imaging the Earth's magnetosphere. IMAGE will utilize the techniques of ultraviolet imaging, neutral atom imaging, and radio plasma imaging to map out global distributions of the electron and proton aurora; the helium ions of the plasmasphere; the ionospheric ion outflow; the medium-energy ions of the near-Earth plasma sheet, ring current, and polar cusp; the high-energy ions of the ring current and trapped radiation belts; and the total plasma density from the ionosphere out to the magnetopause. The imaging perspective is from an elliptical polar orbit with apogee at latitudes from 40° to 90° in the northern hemisphere. For ultraviolet and neutral atom imaging, the time resolution is set by the two-minute spin period of the IMAGE spacecraft, which will be sufficient to track the development of magnetospheric substorms. An important feature of the IMAGE mission is its completely open data set with no proprietary data or intervals. All data, along with software needed for plotting and analysis, will be available within 24 hours of acquisition.     

The Radio Plasma Imager investigation on the IMAGE spacecraft

Radio plasma imaging uses total reflection of electromagnetic waves from plasmas whose plasma frequencies equal the radio sounding frequency and whose electron density gradients are parallel to the wave normals. The Radio Plasma Imager (RPI) has two orthogonal 500-m long dipole antennas in the spin plane for near omni-directional transmission. The third antenna is a 20-m dipole along the spin axis. Echoes from the magnetopause, plasmasphere and cusp will be received with the three orthogonal antennas, allowing the determination of their angle-of-arrival. Thus it will be possible to create image fragments of the reflecting density structures. The instrument can execute a large variety of programmable measuring options at frequencies between 3 kHz and 3 MHz. Tuning of the transmit antennas provides optimum power transfer from the 10 W transmitter to the antennas. The instrument can operate in three active sounding modes: (1) remote sounding to probe magnetospheric boundaries, (2) local (relaxation) sounding to probe the local plasma frequency and scalar magnetic field, and (3) whistler stimulation sounding. In addition, there is a passive mode to record natural emissions, and to determine the local electron density, the scalar magnetic field, and temperature by using a thermal noise spectroscopy technique.     

The IMAGE science and mission operations center

The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) will produce forefront science by quantifying the response of the magnetosphere to the time variable solar wind. It will acquire, for the first time, a variety of three-dimensional images of magnetospheric boundaries and plasma distributions extending from the magnetopause to the inner plasmasphere. The images will be produced on time scales needed to answer important questions about the interactions of the solar wind and the magnetosphere. The IMAGE team will provide open access to all IMAGE data. Thus there will be no proprietary rights or periods. All IMAGE data products will be archived and available to the scientific research community. The IMAGE mission will operate with a near 100% duty cycle with all instruments in their baseline operational modes. A Science and Mission Operations Control Center or SMOC has been developed at the NASA Goddard Space Flight Center (GSFC) to be the main data and command processing system for IMAGE. The IMAGE Level-0 data will be processed into Level 0.5 and Level-1 data and browse products within 24 hours after their receipt of raw data in the SMOC. These data products will be transferred to the NSSDC, for long-term archiving, and posted immediately on the world-wide-web for use by the international scientific community and the public.     

Overview of the image science objectives and mission phases

The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission uses a suite of imaging instruments to investigate the global response of the magnetosphere to changing solar wind conditions. Detailed science questions that fall under this broad objective include plasma processes that occur on the dayside, flanks, and nightside of the magnetosphere. The IMAGE orbit has been carefully designed to optimize the investigation of these plasma processes as the orbit precesses through the magnetospheric regions. We discuss here the phasing of the IMAGE orbit during the two-year prime mission and the relationship between the orbit characteristics and the critical science objectives of the mission.     

The IMAGE Observatory

The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission will be the first of the new Medium-class Explorer (MIDEX) missions to fly. IMAGE will utilize a combination of ultraviolet and neutral atom imaging instruments plus an RF sounder to map and image the temporal and spatial features of the magnetosphere. The eight science sensors are mounted to a single deckplate. The deckplate is enveloped in an eight-sided spacecraft bus, 225 cm across the flats, developed by Lockheed Martin Missiles and Space Corporation. Constructed of laminated aluminum honeycomb panels, covered extensively by Gallium Arsenide solar cells, the spacecraft structure is designed to withstand the launch loads of a Delta 7326-9.5 ELV. Attitude control is via a single magnetic torque rod and passive nutation damper with aspect information provided by a star camera, sun sensor, and three-axis magnetometer. A single S-band transponder provides telemetry and command functionality. Interfaces between the self-contained payload and the spacecraft are limited to MIL-STD-1553 and power. This paper lists the requirements that drove the design of the IMAGE Observatory and the implementation that met the requirements.     

Early results from the ISEE electron density experiment

The ISEE-1 and 2 spacecraft contain two complementary experiments to measure the ambient electron density by radio techniques: a propagation experiment which measures the integrated electron density between ISEE-1 and 2, and a resonance sounder which measures the electron density in the vicinity of ISEE-1, and also provides AC electric field data. These experiments have been described elsewhere (Harvey et al., 1978). Results from these two experiments are presented here for the first time. The propagation experiment permits high time resolution studies of density fluctuations in the solar wind and magnetospheric frontier regions. The sounder experiment has detected for the first time plasma resonances in the solar wind and in the Earth's magnetosheath, as well as in the regions of the magnetosphere where resonances have already been observed by the spacecraft GEOS-1. We present here a preliminary review of the different types of electric field noise observed in the solar wind and magnetosheath, and discuss their relationship to the measured plasma density.CRPE/CNET, 92131 Issy-les-Moulineaux, France.     

Extreme Ultraviolet Imager Observations of the Structure and Dynamics of the Plasmasphere

The IMAGE Extreme Ultraviolet Imager (EUV) provides our first global images of the plasmasphere by imaging the distribution of He⁺ in its 30.4-nm resonance line. The images reveal the details of a highly structured and dynamic entity. Comparing EUV images and selected in-situ observations has helped to validate the remote sensing measurements. The brightness in the EUV images is heavily weighted by the He⁺ density near the plane of the magnetic equator, but two lines of evidence emphasize that the features seen by EUV extend far from the equator, and in at least some cases reach the ionosphere. Certain features and behaviors, including shoulders, channels, notches, and plasma erosion events, appear frequently in the EUV images. These are keys to understanding the ways that electric fields in the inner magnetosphere affect the large and meso-scale distribution of plasma, and their study can elucidate the mechanisms by which the solar wind and interplanetary magnetic field couple to the inner magnetosphere.     

The Lunar Radar Sounder (LRS) Onboard the KAGUYA (SELENE) Spacecraft

The Lunar Radar Sounder (LRS) onboard the KAGUYA (SELENE) spacecraft has successfully performed radar sounder observations of the lunar subsurface structures and passive observations of natural radio and plasma waves from the lunar orbit. After the transfer of the spacecraft into the final lunar orbit and antenna deployment, the operation of LRS started on October 29, 2007. Through the operation until June 10, 2009, 2363 hours worth of radar sounder data and 8961 hours worth of natural radio and plasma wave data have been obtained. It was revealed through radar sounder observations that there are distinct reflectors at a depth of several hundred meters in the nearside maria, which are inferred to be buried regolith layers covered by a basalt layer with a thickness of several hundred meters. Radar sounder data were obtained not only in the nearside maria but also in other regions such as the farside highland region and polar region. LRS also performed passive observations of natural plasma waves associated with interaction processes between the solar wind plasma and the moon, and the natural waves from the Earth, the sun, and Jupiter. Natural radio waves such as auroral kilometric radiation (AKR) with interference patterns caused by the lunar surface reflections, and Jovian hectometric (HOM) emissions were detected. Intense electrostatic plasma waves around 20 kHz were almost always observed at local electron plasma frequency in the solar wind, and the electron density profile, including the lunar wake boundary, was derived along the spacecraft trajectory. Broadband noises below several kHz were frequently observed in the dayside and wake boundary of the moon and it was found that a portion of them consist of bipolar pulses. The datasets obtained by LRS will make contributions for studies on the lunar geology and physical processes of natural radio and plasma wave generation and propagation.     

Cusp dynamics and ionospheric outflow

One of the IMAGE mission science goals is to understand the dayside auroral oval and its dynamic relationship to the magnetosphere. Two ways the auroral oval is dynamically coupled to the magnetosphere are through the injection of magnetosheath plasma into the magnetospheric cusps and through the ejection of ionospheric plasma into the magnetosphere. The ionospheric footpoints of the Earth's magnetospheric cusps are relatively narrow regions in invariant latitude that map magnetically to the magnetopause. Monitoring the cusp reveals two important aspects of magnetic reconnection at the magnetopause. Continuous cusp observations reveal the relative contributions of quasi-steady versus impulsive reconnection to the overall transfer of mass, energy, and momentum across the magnetopause. The location of the cusp is used to determine where magnetic reconnection is occurring on the magnetopause. Of particular interest is the distinction between anti-parallel reconnection, where the magnetosheath and magnetospheric field lines are strictly anti-parallel, and component merging, where the magnetosheath and magnetospheric field lines have one component that is anti-parallel. IMAGE observations suggest that quasi-steady, anti-parallel reconnection is occurring in regions at the dayside magnetopause. However, it is difficult to rule out additional component reconnection using these observations. The ionospheric footpoint of the cusp is also a region of relatively intense ionospheric outflow. Since outflow also occurs in other regions of the auroral oval, one of the long-standing problems has been to determine the relative contributions of the cusp/cleft and the rest of the auroral oval to the overall ionospheric ion content in the Earth's magnetosphere. While the nature of ionospheric outflow has made it difficult to resolve this long-standing problem, the new neutral atom images from IMAGE have provided important evidence that ionospheric outflow is strongly controlled by solar wind input, is `prompt' in response to changes in the solar wind, and may have very narrow and distinct pitch angle structures and charge exchange altitudes.     

Fluctuating magnetic fields in the magnetosphere

The study of Extremely-Low-Frequency (ELF) and Very-Low-Frequency (VLF) waves in space has been intensively pursued in the past decade. Search coil magnetometers, magnetic loop antennas, and electric dipole antennas have been carried on board many spacecraft. The measurements performed by these instruments have revealed a multitude of wave phenomena, whose study in turn is providing a wealth of information on the physics of the magnetospheric and ionospheric plasma. Two classes of wave phenomena are observed: whistlers and emissions. The observed whistler phenomena include: multiple hop ducted whistlers, ion-cutoff whistlers, ion cyclotron whistlers, subprotonospheric whistlers, magnetospherically reflected whistlers and walking trace whistlers.The emissions observed at high altitudes near the magnetic equator differ in many respects from those observed at low altitudes near the ionosphere. At high altitudes, inside the plasmasphere ELF hiss is the dominant emission and outside the plasmasphere chorus is the dominant emission. Also seen is a sub-LHR hiss band in the outer plasmasphere near the equator, and high pass noise and broadband noise in the outer nightside magnetosphere. At low altitude both ELF hiss and chorus are present but, here, ELF hiss is the dominant emission even outside the plasmasphere. Additional emissions, specific to low altitudes, such as VLF hiss and LHR noise are also observed. Although the observations of these phenomena by spacecraft have been complemented by many ground-based and rocket borne studies as well as by spacecraft observations of man-made signals, this paper reviews only satellite observations of signals of natural origin.     

Observations of low energy magnetospheric plasma outside the plasmasphere

After some introductory discussions about morphological concepts and limitations of various measurement techniques, existing low energy plasma data, orginating primarily from the GEOS, Dynamics Explorer, and Prognoz spacecraft, is described and discussed. The plasmasphere measurements are not included (but for some observations of plasmasphere refilling). It is finally concluded that we are very far from a complete picture of the low-energy plasma component in the magnetosphere and that this problem has to be given high priority in planning payloads of future space plasma physics missions.     

Dayside Proton Aurora: Comparisons between Global MHD Simulations and IMAGE Observations

The IMAGE mission provides a unique opportunity to evaluate the accuracy of current global models of the solar wind interaction with the Earth's magnetosphere. In particular, images of proton auroras from the Far Ultraviolet Instrument (FUV) onboard the IMAGE spacecraft are well suited to support investigations of the response of the Earth's magnetosphere to interplanetary disturbances. Accordingly, we have modeled two events that occurred on June 8 and July 28, 2000, using plasma and magnetic field parameters measured upstream of the bow shock as input to three-dimensional magnetohydrodynamic (MHD) simulations. This paper begins with a discussion of images of proton auroras from the FUV SI-12 instrument in comparison with the simulation results. The comparison showed a very good agreement between intensifications in the auroral emissions measured by FUV SI-12 and the enhancement of plasma flows into the dayside ionosphere predicted by the global simulations. Subsequently, the IMAGE observations are analyzed in the context of the dayside magnetosphere's topological changes in magnetic field and plasma flows inferred from the simulation results. Finding include that the global dynamics of the auroral proton precipitation patterns observed by IMAGE are consistent with magnetic field reconnection occurring as a continuous process while the IMF changes in direction and the solar wind dynamic pressure varies. The global simulations also indicate that some of the transient patterns observed by IMAGE are consistent with sporadic reconnection processes. Global merging patterns found in the simulations agree with the antiparallel merging model, though locally component merging might broaden the merging region, especially in the region where shocked solar wind discontinuities first reach the magnetopause. Finally, the simulations predict the accretion of plasma near the bow shock in the regions threaded by newly open field lines on which plasma flows into the dayside ionosphere are enhanced. Overall the results of these initial comparisons between global MHD simulation results and IMAGE observations emphasize the interplay between reconnection and dynamic pressure processes at the dayside magnetopause, as well as the intricate connection between the bow shock and the auroral region.     

CLUSTER and IMAGE: New Ways to Study the Earth’s Plasmasphere

Ground-based instruments and a number of space missions have contributed to our knowledge of the plasmasphere since its discovery half a century ago, but it is fair to say that many questions have remained unanswered. Recently, NASA’s Image and ESA’s Cluster probes have introduced new observational concepts, thereby providing a non-local view of the plasmasphere. Image carried an extreme ultraviolet imager producing global pictures of the plasmasphere. Its instrumentation also included a radio sounder for remotely sensing the spacecraft environment. The Cluster mission provides observations at four nearby points as the four-spacecraft configuration crosses the outer plasmasphere on every perigee pass, thereby giving an idea of field and plasma gradients and of electric current density. This paper starts with a historical overview of classical single-spacecraft data interpretation, discusses the non-local nature of the Image and Cluster measurements, and emphasizes the importance of the new data interpretation tools that have been developed to extract non-local information from these observations. The paper reviews these innovative techniques and highlights some of them to give an idea of the flavor of these methods. In doing so, it is shown how the non-local perspective opens new avenues for plasmaspheric research.     

Mapping Magnetospheric Equatorial Regions at Saturn from Cassini Prime Mission Observations

Saturn??s rich magnetospheric environment is unique in the solar system, with a large number of active magnetospheric processes and phenomena. Observations of this environment from the Cassini spacecraft has enabled the study of a magnetospheric system which strongly interacts with other components of the saturnian system: the planet, its rings, numerous satellites (icy moons and Titan) and various dust, neutral and plasma populations. Understanding these regions, their dynamics and equilibria, and how they interact with the rest of the system via the exchange of mass, momentum and energy is important in understanding the system as a whole. Such an understanding represents a challenge to theorists, modellers and observers. Studies of Saturn??s magnetosphere based on Cassini data have revealed a system which is highly variable which has made understanding the physics of Saturn??s magnetosphere all the more difficult. Cassini??s combination of a comprehensive suite of magnetospheric fields and particles instruments with excellent orbital coverage of the saturnian system offers a unique opportunity for an in-depth study of the saturnian plasma and fields environment. In this paper knowledge of Saturn??s equatorial magnetosphere will be presented and synthesised into a global picture. Data from the Cassini magnetometer, low-energy plasma spectrometers, energetic particle detectors, radio and plasma wave instrumentation, cosmic dust detectors, and the results of theory and modelling are combined to provide a multi-instrumental identification and characterisation of equatorial magnetospheric regions at Saturn. This work emphasises the physical processes at work in each region and at their boundaries. The result of this study is a map of Saturn??s near equatorial magnetosphere, which represents a synthesis of our current understanding at the end of the Cassini Prime Mission of the global configuration of the equatorial magnetosphere.     

Magnetospheric and Plasma Science with Cassini-Huygens

Magnetospheric and plasma science studies at Saturn offer a unique opportunity to explore in-depth two types of magnetospheres. These are an ‘induced’ magnetosphere generated by the interaction of Titan with the surrounding plasma flow and Saturn's ‘intrinsic’ magnetosphere, the magnetic cavity Saturn's planetary magnetic field creates inside the solar wind flow. These two objects will be explored using the most advanced and diverse package of instruments for the analysis of plasmas, energetic particles and fields ever flown to a planet. These instruments will make it possible to address and solve a series of key scientific questions concerning the interaction of these two magnetospheres with their environment. The flow of magnetospheric plasma around the obstacle, caused by Titan's atmosphere/ionosphere, produces an elongated cavity and wake, which we call an ‘induced magnetosphere’. The Mach number characteristics of this interaction make it unique in the solar system. We first describe Titan's ionosphere, which is the obstacle to the external plasma flow. We then study Titan's induced magnetosphere, its structure, dynamics and variability, and discuss the possible existence of a small intrinsic magnetic field of Titan. Saturn's magnetosphere, which is dynamically and chemically coupled to all other components of Saturn's environment in addition to Titan, is then described. We start with a summary of the morphology of magnetospheric plasma and fields. Then we discuss what we know of the magnetospheric interactions in each region. Beginning with the innermost regions and moving outwards, we first describe the region of the main rings and their connection to the low-latitude ionosphere. Next the icy satellites, which develop specific magnetospheric interactions, are imbedded in a relatively dense neutral gas cloud which also overlaps the spatial extent of the diffuse E ring. This region constitutes a very interesting case of direct and mutual coupling between dust, neutral gas and plasma populations. Beyond about twelve Saturn radii is the outer magnetosphere, where the dynamics is dominated by its coupling with the solar wind and a large hydrogen torus. It is a region of intense coupling between the magnetosphere and Saturn's upper atmosphere, and the source of Saturn's auroral emissions, including the kilometric radiation. For each of these regions we identify the key scientific questions and propose an investigation strategy to address them. Finally, we show how the unique characteristics of the CASSINI spacecraft, instruments and mission profile make it possible to address, and hopefully solve, many of these questions. While the CASSINI orbital tour gives access to most, if not all, of the regions that need to be explored, the unique capabilities of the MAPS instrument suite make it possible to define an efficient strategy in which in situ measurements and remote sensing observations complement each other. Saturn's magnetosphere will be extensively studied from the microphysical to the global scale over the four years of the mission. All phases present in this unique environment — extended solid surfaces, dust and gas clouds, plasma and energetic particles — are coupled in an intricate way, very much as they are in planetary formation environments. This is one of the most interesting aspects of Magnetospheric and Plasma Science studies at Saturn. It provides us with a unique opportunity to conduct an in situ investigation of a dynamical system that is in some ways analogous to the dusty plasma environments in which planetary systems form. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dynamical Simulations of Plasmapause Deformations

Dynamical simulations have been developed at IASB-BIRA to model the deformations of the plasmasphere during geomagnetic substorms and other variations in the level of geomagnetic activity. The simulations are based on the mechanism of plasma instability and use the empirical Kp-dependent electric field E5D. The results of the simulations are compared with IMAGE observations that provide the first global comprehensive images of the Earth’s plasmasphere. The predicted plasmapause positions correspond generally rather satisfactorily with the EUV observations. The plasmasphere is rather extended in all MLT sectors during quiet periods. During or just after geomagnetic substorms, the plasmaspause is sharper and becomes closer to the Earth in the night sector. Periods of enhanced geomagnetic activity are associated to the formation of plumes that rotate with the plasmasphere. The simulations reproduce the formation and the motion of these plumes, as well as the development of other structures like shoulders observed at the plasmapause by EUV on IMAGE.